I am trying to figure out the size of the supply conductors and OCPD for an existing industrial control panel. (From Bus Plug to Machine Disconnect) However, when I write it down on paper. I am noticing a significant discrepancy. So naturally I pump the brakes and I am assuming I am doing this incorrectly.
The machine I am looking at. I have:
1.) VFD supplying a 15HP 460V motor. Table 430.250 gives me a FLA of 21A
2.) 12KW 480V 3ph Resistive Heating Element (12,000W / 480 / 1.732) = 14.43A
3.) 5kVA transformer 1ph (5,000 / 480) = 10.4A
NEC 670.4(A) 125% FLC rating of resistance heating loads + 125% FLC of Highest Rated Motor + 100% FLC of all other:
Motor (21*1.25) + Heater (14.43*1.25) + Transformer(10.4)
26.25+18.04+10.4 = 54.69A Size of Supply Conductor 6AWG (60*C)
NEC 670.4(C) Largest Rated or Rating of Branch Circuit Short Circuit Protective and Ground Fault Device + 125% of FLC of all resistance heating loads + 100% of all other
VFD(45A) + Heater (14.43*1.25) + Transformer (10.4)
45 + 18.04 + 10.04 = 73.08A Size of Supply OCPD
So as you can see. I am misunderstanding what the code book is getting at with this, because I wouldn't think I would be justified putting 70A Fuses on 6AWG (55A) conductors. It just seems wrong. Why wouldn't the code be written to just 670.4(A) and leave it at that. Then you would size the OCPD to the supply conductors and that's it. Why does the code go on to include 670.4(C)? I am guessing that I am misunderstand when do we use 670.4(C). Or do we use them both?
Thanks in advance
The machine I am looking at. I have:
1.) VFD supplying a 15HP 460V motor. Table 430.250 gives me a FLA of 21A
2.) 12KW 480V 3ph Resistive Heating Element (12,000W / 480 / 1.732) = 14.43A
3.) 5kVA transformer 1ph (5,000 / 480) = 10.4A
NEC 670.4(A) 125% FLC rating of resistance heating loads + 125% FLC of Highest Rated Motor + 100% FLC of all other:
Motor (21*1.25) + Heater (14.43*1.25) + Transformer(10.4)
26.25+18.04+10.4 = 54.69A Size of Supply Conductor 6AWG (60*C)
NEC 670.4(C) Largest Rated or Rating of Branch Circuit Short Circuit Protective and Ground Fault Device + 125% of FLC of all resistance heating loads + 100% of all other
VFD(45A) + Heater (14.43*1.25) + Transformer (10.4)
45 + 18.04 + 10.04 = 73.08A Size of Supply OCPD
So as you can see. I am misunderstanding what the code book is getting at with this, because I wouldn't think I would be justified putting 70A Fuses on 6AWG (55A) conductors. It just seems wrong. Why wouldn't the code be written to just 670.4(A) and leave it at that. Then you would size the OCPD to the supply conductors and that's it. Why does the code go on to include 670.4(C)? I am guessing that I am misunderstand when do we use 670.4(C). Or do we use them both?
Thanks in advance
. So thinking on it over time, I think I have come to the following conclusions: